Honda CB125 TD-C carb issues. I've run out of ideas.

[threeeyedjoke...]

Help! I have a Honda CB125TD-C and am having what I think are carb issues that I can’t solve.

First the bike
The bike is a 1983 Honda CB125 TD-C (a Superdream). It’s been café racer’ed, so looks a beaut. Striped right back, sand blasted and freshly painted, new seat, low handle bars – you get the picture.

It has

2 x Keihlin PD64A carbs (slide)
A new petrol tank of a different, bigger bike (not relevant to the situation, but I thought I’d mention it just in case)
No air box. That’s gone. Replaced with 2 x 50mm pod filters - new.

It sat for over a year, and then I bought it.

So first off I had to clean out the petrol tank as it was rusty inside (as a result of doing that I lost the petcock union nut, which after 2 hours of dangling myself head first down a drain sifting through slurry trying to find it again, I had to purchase a new one from the US on ebay. Anyway….). Next job was to clean out the carburettor – and it was pretty dirty in there. I’ve also fitted new spark plugs and new pod filters.

I then cleaned the carb again to double check.

I have checked that the throttle slide needle clip is not in the middle of the 5 grooves, but one down from the top.

I know the flex link on the choke mechanism between the 2 carbs is a crap design, so I’m making sure by hand that its closed.

Now the problems

I have three issues that I can not solve. I don’t know if they are related or not.

1. The spark plugs get blackened up very quickly – like 2 minutes of idling. I have tried everything I can think of to lean up the mixture. I have played with the air/fuel screw on both carbs. Even when they are FULLY screwed in it’s still blackening up. I’ve checked the seals in and out of the carb (on the boots) and there’s no leak.

2. Next, the bike has erratic rev’s. When idling it moves from 1,000 to 1,500 without me touching anything. It sometimes misfires, and does a spitting sound (might be because of the blacked up spark plugs). If I take the rev’s up to 5,000 or more for a little while, then let the throttle go, the revs come down but sometimes stop around 3,000 – before dropping back down again after 5-10 seconds. The throttle itself has definitely dropped back down at this point on the handle level.

3. One cylinder gets a lot hotter an lot quicker than the other – the left hand one as you sit on the bike is the hotter one.

The bike has the super unleaded/ 98 unleaded fuel in at the moment – I don’t think that would be the cause, but, again, I mention it just in case.

I’d really appreciate any ideas or suggestions as I am running out of my own ideas.

[jaffa90]

Being over 30 years young i would check the valve clearances and then a compression check.
Then i would check that the chokes are closing and make sure the fuel levels do not vary with faulty shut off needle valves.

[Johnnythefox]

put the air box back on.

[redeem ouzzer]

Clean the carbs properly (with an ultrasonic bath), then be prepared for lots of fiddling to get it run without an airbox.
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Be a REAL MAN!

[Tankie]

balance the carbs , clean the air filter , set the idle mixture screws to about 1.5 turns out
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Always learning, everyday is a school day

[P.]

[Teflon-Mike]

Sounds about typical.
A cosmetc make-over, some notions of cafe-sxrambler type stuff going on, and "Oooh! It dont run right... lets blame carbs...."

1/ These motors are not particularly carb sensitive; they dont like pod filters much, but meh! They run pretty hapily without air-box or filter, and pods do a lot to strangle them, nipping flow at the mouth... but even with a pair of the smaller PD24's, with a pair of moped sized pots to feed, they aren't restricted much if at all carb-side.

2/ You will never set up a good carb on a crap motor. Believe me I have wasted far to much time trying, and have tried just about all perms of small honda carbs, singles and twins, and muggered about with the jets and needles, carb-cleans and overhaul kits to now not bother.

These old engines have tiny pots, they don't show obvious bore or ring wear on tear down, they show it by refusing to run right.. on such a small over-square motor, they do not make much manifold vacuum, they like sharp rings and good bore to get any decent 'suck' at the carb...

Constantly fouling plug, to ME suggests the thing is burning oil, probably not a lot past the rings, probably past well perished stem-seals...

As matter of course, 'now', I cut to the chase; I pull the motor and tear-down. A Chinese barrel kit, with new rings and gaskets is chaper than a rebore, at last look aprox £120. Head comes off and gets stipped; valves get lapped in, and new stem seals fitted. Then it goes back together on the Chnky barel, attension payed to the cam and cam bearings, rockers and state of the likely chewed up tappets. Cam gets critically timed in on rebuld, and hint the Chinky replacements are crap... cap bearings are worth the swap though. A-N-D the top-tip, IF the cam-chain loops onto the sprocket easily, and you dont have trouble timing it in... then the cam-chains likely fcuked!

That work done, motor back 'in', the exhaust is the first thing to look at before carbs; few OE pipes will have survved by now, I certanly haven't found any! After market Micron is pretty tough, but the Motad I think better, but I have yet to find any system that hasn't got dings in the down-pipes at kerb-height and few that have avoided pin-holing at the lower end of the header, usually on the outside of the bend.

Again; you are on a fools errand trying to set up a shit-motor on a questionable exhaust, even a decent moto with a crappy pipe... so make sure the pipe is good, and you dont either have two or three crush washers squashed into the port-flange, or no washer at all, omitted by some numpty believing it the mystical 'restrctor'....

Replacing the exhaust studs whilst the head is loose is also a good precaution, and wonderful ad to actually gettng the exhaust to seat, and seal!! But nspect closely whilst 'off'.. not unknown for heads to be butchered by folk with errant hack-and-wrecker drill trying to drill out old ones.. also not unknown to find any odd assortment of M6 olt used in substitution, or for the exhaut ports to be botchred with imperial roofbolts or the like used as self tappers in the alloy, or for M8's to have been forced into stripped holes, and flanges to have been drilled to take them!

With good head, barel, and 'zorst, set the carb up to the book, and away you go.....

Beware carbs overhauled with chinky overhaul kits; they often have the wrong sized jets in them, some marked with mikuni sizing rather tha Keihin, which ca cause some consternation, but most are CG derived and contain the wrong float valce needle, slide needle and wrong sized jets!!! Pilot air-screw however IS useful, especially as the very fine needle on the end is want to sieze in the carb-body and shear when you try adjust.... Remove check and when cleaning check carefully that you do HAVE a fine needle on the end of the air-screw.. A-N-D there's no 'old' sheared needle end left in the carb body!

It's all pretty easy to do, just takes time and a bit of wallet gulping to man up the cash for the barrel kit. and gonad appraisal to get the motor out and venture into the oil-covered bowels B-U-T one over that mental hurdle, its a hell of a lot more likely to actually 'work; and work well, and long-term be less work, hassle and cash, than faffing around with the bits outside the motor hoping that some-how problems will 'adjust' out.

Your call.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Robby]

Balance the carbs first. Maybe just do it by eye, to get them close-ish.

So that's carbs out, hold them up to the light and look through the butterflies so you can see little rim of light through each one. You want them to both be open the same amount.

If they are really out of balance, then one butterfly is closed whilst the other is open. This makes the bike behave like it has an air leak, which is why the revs are taking so long to get back to idle.

Don't worry about the state of the plugs now. The mixture can be very rich at idle, and I'm assuming this bike has spent most of its time at idle recently.

Do put the mixture screws back to where they should be - or even an extra half turn open. Mixture screws only affect the mixture at idle and really tiny throttle openings. Everything else is controlled by the jets and needles.

Worth making sure the float height is correct.

So in short, clean them properly - if you haven't had the mixture screws and needle valve filters out while cleaning, they'll be full of shit. Then reset to factory settings, including balance.

[P.]

With the float heights, I'd really REALLY recommend making sure the rubber on the bowl is still soft, they tend to get dry and flatten out, they then stop sealing well

[threeeyedjoke...]

Thanks everyone - that's a great list of things to do.

Here's what I'm going to do:

Check the carb balance - at least by eye
Check the float heights
Check valve clearances
Do a compression test

However, I reckon Teflon-Mike's going to be right, and so I'll 'pull the motor and tear down'. I bought the bike to learn how it all works, so why not. So Mike - I have a few questions. What you've said is no doubt right, but some of it doesn't mean much to me, as I'm the thick boy at the back of the class.

1. Is this the Chinese barrel kit? https://www.wemoto.com/bikes/honda/cb_125_tdc_tde_superdream/82-87/picture/barrel_and_piston_-_assembly/

2. What do you mean by a) Head comes off and gets stipped; and b) valves get lapped in?

3. Which part of the Chinese kit is dodgy and you think I shouldn't put in?

I suspect I'll be back once I've done that and start to look at the exhaust!

Thanks again.
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Bikes owned: Honda CB125TD-C, and a very rusty 1963 Vespa 125 that I ride everyday around London, which that goes like the stink

[Teflon-Mike]

Robby - CB125's use old fashioned slide carbs, not CV's; the only butterfly is the choke, not the throttle control. They are also individually mounted, and linked only by the flex-link twixt the choke butterfly's and the Siamese cable that lifts the slides.

Paddy.... the float needles in the old PD's are a bit flakey, they have a habit of sticking in the rebate; they do have a habit of flooding, but usual symptom is they pee out of the vent hole in the actual carb-body, just behind the choke cable bracket. Actually quite common to have peeing carbs on first start or after even a short lay-up when the float needles 'stick'... sometimes they'll stop if you give them a tap with a screwdriver handle, but most often not; but give one a good thrashing and drain the float bowl and make the needle go up and down in full travel a couple of times twixt bends, and they'll usually sort themselves out!

I have never really had issue with the actual float-bowl gaskets going hard... have had issue with flakey tolerance Chinky replacements though, which have been worse than rather nibbled originals! And a cornflake-packet gasket, can see the rubber seal binned... just means you need replace the CFP Gasket if you drop the float bowl....

[P.]

[Rogerborg]

[threeeyedjoke...]

Before I get stuck in properly, a couple of questions:

Before I refit the new barrel, do I need to hone it? If so, I don't have a honing tool so should I a) shut up and stump up the cash for one, b) take it to a Honda mechanic and ask them to do it, or c) home make one out of sandpaper and a drill?

In one of Teflon Mike's replies, he said "the head comes off and gets stipped". What does that mean?! Or is it a typo for stripped?

Assuming it all goes well (gulp), I'm left with sorting the carbs and exhaust. I'll be back with more questions no doubt at that stage.
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Bikes owned: Honda CB125TD-C, and a very rusty 1963 Vespa 125 that I ride everyday around London, which that goes like the stink

[stinkwheel]

My CB125TDC carburetted perfectly on a straight through pipe and pod filter when I fitted just one of the original carbs on factory jetting using a CM125 2-1 inlet manifold.

It went faster too because it was perfectly balanced.

Just saying...
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Teflon-Mike]

[threeeyedjoke...]

Hi. I've got the barrel off, and have a few queries:

1. There's 3 hollow dowels that are stuck, two in the barrel that I am replacing. Any tips on how to get them out without damaging them?

2. The pistons are really blacked up - is that normal, or does that mean replacing the barrel and pistons is the right thing to be doing?

3. The Haynes manual says to catch the pistons as you remove the barrel, with the pistons at TDC. But to be at TDC, one piston is up and the other down. Am I being stupid?

Thanks for any advice you can give


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Bikes owned: Honda CB125TD-C, and a very rusty 1963 Vespa 125 that I ride everyday around London, which that goes like the stink

[Teflon-Mike]

[threeeyedjoke...]

I've got the engine back together, and on the bike. When I first started it, it started quite easily and ran for about 30 seconds, badly, but the engine ticked over. I noticed only one exhaust getting hot though.

And now it won't go any more.

The carbs are clean. There's a good spark. Petrol is coming out of the bottom of the carb freely if I unscrew the drain. The pod filters are new.

Help! What should I do? It's bleedin frustrating after the blood sweat and tears of replacing the barrel and pistons for the engine to not start now. Arrrrrrggggh.


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Bikes owned: Honda CB125TD-C, and a very rusty 1963 Vespa 125 that I ride everyday around London, which that goes like the stink

[Teflon-Mike]

First, don't panic!

Next Very vert normal for these little things to start up on just one cylinder, and for only one exhaust to get hot at first.
- Check you do have sparks on both cylinders.
- Smell the plug on the cold header pot's plug.. is it wet/smell petrol?

It's an e-start only engine; and it uses a 'magneto' style ignition; the electric for the sparks, doesn't come from the battery, but direct from the generator, so motor has to spin over pretty well to get a decent spark. If the battery is weak, and or the starter-motor is lazy, then crank may not spin quickly enough to give a good spark.

You will likely see sparks when you take them out to test, but electric can jump the gap pretty easily in 'free' air; under compression in the cylinder, the piston is squashing 62cc of air into a 6cc combustion chamber, so you have 10x the amount of air molecules between the spark-plug electrodes for the electric to try and jump through, and it has a slightly harder time.

Ie you will often 'see' sparks when you take plugs out to check, but it's not guaranteed you'll still have them when you put plug back in.

Next up; on first fire fault finding... you will likely drain the battery pretty hard on cranking, so when it conks and you try re-start, it lacks the 'oomph' to do so.

Tip: I tend to use a boost-box or car battery for first-fire and fault-finding; a) to save 'killing' little bike battery on repetitive re-starts b) to save having to keep recharging it between start-up attenpts

On first fire... if you followed advice for first-fire lube, and used two-stroke oil to lube bores ad rings and things, it is very likely that the plugs will soot up burning that off.

Also likely, that if it hasn't 'caught' properly, that the motor is flooding, due to the amount of juice sucked through during start attempts, and or, the prev mentioned sticky floats, and or the choke being on, and or, the choke not properly going 'off' and or the flex-link twixt choke flaps being generally buggeratiouse!

1/ Check the choke link
2/ Pull plugs, burn them off with a blow-torch or heat gun, five minutes n the oven or laid on the gas-ring, to make sure they are 'dry' before leaning the electrodes again, and popping back in the pots.
3/ Make sure battery well charged / use car battery on jumpers for fault-finding starts.
4/ start from scratch... no-choke (and probably no pod-filters) for start up fault-finding.. when it catches, don't be over eager, give it a chance to warm and settle onto both pots.. when it dies.. don't get frustrated.. be patient! Be methodical, work back through the process.

There's a lot of variables from how much choke it needs/is used, how much throttle, which changes as it warms; and those float needles are sticky, you may have to tap the float bowls to get them to play ball, until the motor is bedded in a bit, and the floats have, from going up and down full travel a few times, from on-load thrashing cleared their track and aren't so want to 'stick' at top or bottom and cause flood or famine.

Patience and perseverance is the key
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'